1. Field of the Invention
The present invention relates to an electric power mechanism and, more particularly, to a drive system for driving an electric vehicle by a motor acting as a prime mover.
2. Related Art
An internal combustion engine employing a fossil fuel mainly of gasoline may cause environmental problems such as exhaust gases or noises. The fossil fuel per se has a fear of being exhausted in the near future. In recent years, therefore, there have been developed the electric vehicles which employ electric energy as the driving power source. The motor to be mounted on the electric vehicle has such output characteristics that its torque is substantially constant up to a predetermined number of revolutions (RPM) and is gradually lowered after the predetermined RPM is exceeded.
When the electric vehicle runs upslope, on the other hand, it demands a high torque at a low speed. For this demand, the motor to be mounted on the electric vehicle has to be of the low-speed high-torque type.
However, the motor of this type has a low maximum RPM and invites a reduction in the output at a high RPM so that it cannot satisfy all the performances demanded by the electric vehicle. Therefore, the electric vehicle has adopted a construction in which a transmission is arranged at the output side of the motor so that the output torque of the motor may be increased or decreased by the transmission and transmitted to the wheels. One example of the electric vehicle drive system, in which a reduction mechanism is arranged at the output side of the motor, is disclosed in Japanese Patent Laid-Open No. 42656/1996 (JP-A-8-42656).
The electric vehicle drive system, as disclosed, is provided with an electric motor, a reduction mechanism arranged at the output side of the electric motor, and a differential arranged at the output side of the reduction mechanism. Moreover, these electric motor, reduction mechanism and differential are arranged on a common axis.
In addition, the reduction mechanism and the differential are constructed of planetary type rollers. Specifically, the reduction mechanism is composed of: a first sun roller coupled to the output shaft of the electric motor; a first ring roller arranged around the outer circumference of the first sun roller; a plurality of first pinion rollers contacting with the first sun roller and the first ring roller; and a first carrier supporting the first pinion rollers rotatably.
On the other hand, the differential is composed of: a second ring roller fixed on the first carrier; a second sun roller arranged in the inner circumference of the second ring roller; a plurality of second pinion rollers contacting with the second ring roller and the second sun roller; and a second carrier supporting the second pinion rollers rotatably.
In the inside of the output shaft, there is inserted a righthand drive shaft, which is aligned with a lefthand drive shaft. Moreover, the second sun roller is coupled to the righthand drive shaft, and the second carrier is coupled to the lefthand drive shaft.
According to the conventional system thus far described, the torque of the output shaft of the electric motor is decelerated by the reduction mechanism and transmitted to the differential. The torque thus transmitted to the differential is distributed between the righthand drive shaft and the lefthand drive shaft to drive the electric vehicle.
A difference between the RPMs of the righthand and lefthand drive shafts is absorbed, if made by the resistance between the wheels and the road surface during the run of the electric vehicle, by the function of the differential. Moreover, since all those components of the electric motor, the reduction mechanism, the differential and the righthand and lefthand drive shafts are arranged on the common axis, the space for arranging the parts in a radial direction, i.e., in a direction perpendicular to the axis is minimized to offer an advantage for the size reduction.
In the conventional system, however, there are separately constructed the reduction mechanism for decelerating the output torque of the electric motor and the differential for absorbing the difference in the RPM between the righthand and lefthand drive shafts. This construction increases the number of parts and enlarges the space for arranging the parts in the axial direction. This raises a problem that the drive system is enlarged in size and increased in weight. Moreover, the construction of the reduction mechanism and the differential of the planetary type rollers enlarges the space for arranging the parts in the radial direction and increases the external diameter of the drive system.